In 1923, Coster and von Hevesy[cite]10.1038/111079a0[/cite] claimed discovery of the element Hafnium , atomic number 72 (latin Hafnia, meaning Copenhagen, where the authors worked) on the basis of six lines in its X-ray spectrum. The debate had long raged as to whether (undiscovered) element 72 belonged to the rare-earth group 3 of the periodic table below yttrium, or whether it should be placed